THE CRANIAL NERVES 295 



ance of deglutition, loss of cardiac inhibition and partial 

 paralysis of the sterno-mastoid and trapezius. The loss of 

 voice and disturbance in deglutition are explained by the dis- 

 tribution of the fibers of the eleventh. with the pharyngeal 

 and laryngeal branches of the tenth. The loss of the power 

 of the vagus to inhibit cardiac action is because the fibers of 

 the tenth which convey the inhibitory impulses are received 

 from the spinal accessory. The sterno-mastoid and trape- 

 zius are only partially paralyzed because they receive motor 

 fibers also from the cervical plexus. 



Twelfth Nerve (Hypoglossal). 



Origin. This nerve supplies motion to the tongue. Its 

 apparent origin is by 1015 filaments in the groove between 

 the anterior pyramid of the medulla and the olivary body. 

 Its deep origin is in the floor of the fourth ventricle under 

 the lower border of the fasciculus teres. 



Course and Distribution. The nerve passes through the 

 anterior condyloid foramen in two bundles which unite to 

 form a common trunk below. Running downward in com- 

 pany with the internal carotid artery and internal jugular 

 vein, it reaches a point opposite the angle of the jaw, then 

 runs forward, crosses the external carotid, lies on the hyo- 

 glossus and is continued forward in the genio-hyoglossus 

 to the tip of the tongue. 



It communicates with the tenth, sympathetic, .first and sec- 

 ond cervical and the lingual branch of the fifth. 



Its branches of distribution are ( I ) meningeal to the dura 

 mater in the posterior fossa of the skull; (2) descendens 

 hypoglossi, which running downward across the sheath of 

 the great vessels, meets branches of the second and third 

 cervical nerves to form a loop from which are supplied the 

 sterno-hyoid, the omo-hyoid and the sterno-thyroid muscles ; 

 (3) thyro-hyoid to the muscle of that name; (4) muscular 

 to the muscular substances of the tongue and to the stylo- 



